Lethal injection is the big one. It used to be that there was a standard multiple drug combination whose purpose was to make death both humane and certain. Obtaining it was no problem, especially in the majority of states where executions are relatively rare. For places like Texas and Florida, they seem a little more commonplace. In any event, even those places got enough to satisfy their needs. For someone to evade or delay execution, it was going to take some kind of legal maneuver.
It has now been some time since the original drug combination became entirely unavailable for executions. Since then, states have become more and more desperate for anything that will get the job done. Necessity is the mother of invention, as they say, and the states are getting very creative with drugs that never have been used for such a thing. Executions are getting delayed just because they can't come up with anything. It may come to the point where another execution method must be found, or else the death penalty will end. I don't expect the latter, but it should be interesting to see how things play out.